"Today's spear phishing is not only more prevalent but also much more technically proficient," said Dave Jevans, chairman of the Anti-Phishing Working Group (APWG), an industry association dedicated to fighting online identity theft. Jevans is also the founder and chairman of IronKey, a Sunnyvale, Calif. security company.

"They're not going for a password, anymore, they're getting people to install crimeware on their computers," said Jevans.

Like the more common phishing, spear phishing attacks are launched as emails that try to con the recipient into clicking a link that leads to a malicious Web site. Those sites can take almost infinite forms, from fake account log-in screens to ones that tout a software upgrade to widely-used software, such as Adobe Flash.

In the second scenario, the file is not as advertised, but instead is attack code that infects the computer, giving criminals access to that machine -- and through it, others -- or to confidential information, like account passwords obtained by secretly monitoring the PC's keystrokes.

According to reports by the likes of Bloomberg, the IMF suspected that a phishing attack against one of its workers planted malware on a machine, which was then presumably used to scout the network for data to steal.

But the IMF incident was only the most recent in a series of specialized attacks this year aimed at targets from the Oak Ridge National Laboratory and the French foreign ministry to Google's Gmail.

All have one thing in common: They relied on spear phishing to fool users into installing malware or revealing account information.

The difference between phishing and spear phishing is while the former floods thousands or even millions of inboxes, the latter targets a small group of previously-identified people, sometimes only a handful who work at the same company or in the same organization.

It's like the difference between two letters asking for a loan: One addressed to "Occupant," the other from a best friend.

A key element of spear phishing is the reconnaissance hackers conduct before they launch their attacks, using the information they find on individuals to personalize the messages or to spoof the sending address of a colleague.

"They're doing a lot more legwork," said Jevans. "There's a lot more data on the Internet, on Facebook, on LinkedIn, that make these emails highly believable. And the malware that they're installing continues to evade antivirus software."

"Social engineering has always been around," said Haley, referring to the term that describes the hacker strategy of manipulating victims into divulging information or doing something actually against their interest, like downloading malware. "But now [criminals] have perfected it."

Haley cited the example of the targeted attacks against Gmail users that Google said last week it had disrupted. Those attacks, which ran for months, were aimed at senior U.S. and South Korean government officials, military personnel, Chinese activists and journalists.

"You could have looked pretty hard [at the phony Gmail log-in screen] and not found any problems," said Haley.

"We used to say, 'Those stupid users, they're falling for obvious attacks,' but we can't do that anymore, maybe we shouldn't have done that in the first place," said Haley. "The social engineering [in targeted attacks] has reached a point where it's pretty incredible."

But unlike Jevans, Haley isn't ready to give hackers' spear phishing expertise all the credit for the rash of big-name break-ins this year.

Haley argued that while spear phishing has become more insidious, it hasn't become more frequent. But the fact that more companies and organizations are willing to retroactively acknowledge an attack or proactively disclose one has created that perception.

He pointed to Google's very public disclosure in early 2010 that it had been hacked, allegedly by Chinese attackers, for kick-starting the trend. "Google seems to be the first to come out and talk publicly [about an attack]," Haley said. "Credit to them."

But he also traced the change to Stuxnet, the worm found by researchers in 2010 that most experts believe was built as a digital weapon, then aimed at Iran's nuclear program.

"Stuxnet made us more aware of these types of attacks and the stakes of those attacks," Haley said. "It was no longer a discussion about the theoretical, but made everyone realize that these kind of attacks could be very serious."

Some security experts have said that the IMF attack smelled of state-sponsored hacking -- attacks that were either government-run or government-financed. While Jevans and Haley were willing to rule that out, they noted that sophisticated spear phishing was well within the capabilities of cyber crime gangs.

"Although it's plausible, I'm not sure I buy it," said Jevans. "Governments certainly don't have more talent at their disposal than do the more sophisticated crimeware gangs."

As for defending against spear phishing, neither expert had a clear-cut answer. "There's no silver bullet," said Jevans, though he noted that isolating browsers and email clients in anti-malware "sandboxes" shows promise.

"There's not one thing that will stop this," echoed Haley. "All it takes is one user who goes into their junk mail folder and clicks on a link. We have to continue developing technical solutions, but if we ignore user education about targeted attacks we do ourselves a disservice."